Heat Recovery Module

ABSTRACT

A module for heat recovery from exhausted flue gas in a steam generator is described comprising a flue gas outlet conduit defining a flow path for flue gas from a flue gas outlet of a steam generator to a flue gas conduit junction; a first flue gas conduit defining a flow path for flue gas from the junction to a first air heater; a second flue gas conduit defining a flow path for flue gas from the junction to at least one high pressure and at least one low pressure process liquid economiser. A steam generator using the module, a method implementing the flow principles embodied in such a module, and a method of incorporation of such a module into a steam generator, especially by retrofit, are also described.

This invention relates to a module for heat recovery from exhausted flue gas in a steam generating process involving a steam generator such as a boiler, which could form part of a power plant using steam driven turbine generators and a condensate and feed heating system able to extract steam from inner-stages of the turbine to heat the condensate and feedwater to boost cycle efficiency. This invention also relates to steam generation systems, and a method for same.

In a steam generating process, there are generally one or more steam generators, one example of which is a boiler, which can be fired by various types of fuels, such as oil, gas, biomass and coal. Boilers for steam generation are well known, and can form part of a power plant using the steam to drive one or more steam turbine generators.

From the steam generator(s) there is an exhausted flue gas which conventionally provides heating for combustion air. Air pre-mixed with fuel before ignition is commonly termed ‘primary air’, and air for supporting combustion after ignition is commonly termed ‘secondary air’. For coal- or biomass-fired boilers, the primary air may form a separate stream as its pressure may be much higher than that of secondary air. Combustion air including primary air and secondary air can be (pre-)heated by use of regenerative air heaters.

Conventionally, the primary and secondary air streams are heated by heat exchange with the flue gas stream in gas-to-air heat exchangers such as a regenerative gas air heater. However, the heat capacity of flue gas is much higher than the heat capacity of the air in the conventional arrangement of gas-to-air heat exchangers, making the heat exchange process inefficient.

According to a first aspect of the present invention, there is provided a module for heat recovery from exhausted flue gas from a steam generator in a steam generating process comprising:

(a) a flue gas outlet conduit defining a flow path for flue gas from a flue gas outlet of a steam generator to a flue gas conduit first junction;

(b) a first flue gas conduit defining a first flow path for flue gas from the first junction to a first air heater; and (c) a second flue gas conduit defining a second flow path for flue gas in parallel with the first flue gas conduit from the first junction to at least two process liquid economisers comprising at least one high pressure economiser and at least one low pressure economiser.

By using process liquid economisers comprising at least one high pressure economiser and at least one low pressure economiser, it is possible to better match the heat capacity of the flue gas with the air flow in the first air heater and the process liquid in the process liquid economisers. The backend temperatures may drop significantly. This also reduces the energy losses of the heat transfer process. The efficiency of the overall steam generating process will increase.

The first air heater may comprise one or more first air heaters being in series, parallel or both. The first air heater preferably comprises at least a regenerative gas air heater able to heat one or more air streams, such as: at least one secondary air stream, at least one primary air stream, or both. This could include a multi-sector regenerative air heater.

Regenerative air heaters are compact and effective gas-to-air heat exchangers for heat recovery from exhausted flue gas. The total weight and volume of the air heaters could be minimised by using regenerative air heaters.

In one embodiment of the present invention, the first air heater comprises at least a gas secondary air heater, preferably for a secondary air stream intended to be provided to a steam generator such as a boiler, in the steam generating process. Generally, it is desired to raise the temperature of secondary air to a temperature suitable for the combustion process, which can be greater than 200° C. or greater than 300° C., as known in the art.

Each of the high pressure and the low pressure process liquid economisers may comprise one or more economisers being in series, parallel or both. Embodiments of the present invention as described hereinafter may apply to a single process liquid economiser, each of a plurality of process liquid economisers, or be variable across a plurality of process liquid economisers.

In accordance with the present invention, the heat recovery module comprises at least one high pressure economiser and at least one low pressure economiser. The terms “high pressure” and “low pressure” are known to the person skilled in the art in relation to steam generating processes, especially involving a steam generator such as a boiler. Generally, they relate to the relative pressure of a stream upstream and downstream respectively from a feed pump.

The process liquid may be provided in one or more streams. A plurality of process liquid streams may be provided in series, parallel or both, and optionally from a single source or a plurality of sources being the same or different. The properties and composition of each such stream may be same or different.

The process stream may be provided in one or more process liquid circuits, optionally a plurality of separate circuits, being separate or connected, each circuit optionally passing also through any separate primary air heat exchanger to exchange heat between any primary air stream of the steam generating process and a process liquid stream.

The process liquid may be any liquid or combination of liquids useable for heat exchange, including water, ammonia, alcohols, hydrocarbons and the like. Preferably, the process liquid is wholly or substantially water, optionally including one or more additives or other minor components known in the art.

In another embodiment of the present invention, the process liquid is feedwater for a steam generator. Such a steam generator may be a boiler, optionally comprising one or more boilers, and optionally including an integral steam generator economiser as known in the art. With the use of feedwater as the process liquid, heat can be transferred from exhausted flue gas to feedwater directly which would minimise the energy losses of the heat transfer process.

Such feedwater may be provided directly or indirectly from a feedwater stream to be processed by one or more steam generators of the steam generating process of the present invention. Preferably, a portion of such a feedwater stream is provided as the process liquid for the present invention. Such a portion may be provided as the full feedwater stream, or preferably as a slip stream of such a feedwater stream, such a slip stream generally being a minor portion of the full feedwater stream.

According to another embodiment of the present invention, the feedwater is provided from the feedwater stream in the steam generating process between the steam condenser and the steam generator economiser.

Thus, the module of the present invention may further comprise one or more first process liquid conduits defining one or more flow paths for directing process liquid from a feedwater stream to the process liquid economisers.

The module of the present invention may also further comprise one or more second process liquid conduits defining one or more flow paths for directing process liquid from the process liquid economisers to a feedwater stream.

According to another embodiment of the present invention the flue gas first conduit junction comprises a set of proportioning dampers to divide the flue gas in use into the first and second flue gas paths to best match the heat capacity of the second flue gas path with the heat capacity of airflow in the first air heater.

The module of the present invention may also comprise:

(d) a third flue gas conduit defining a flow path for flue gas from the first air heater to a second junction; (e) a fourth flue gas conduit defining a flow path for flue gas from the process liquid economisers to a second junction; and (f) a fifth flue gas conduit defining a flow path for flue gas from the second junction.

In a possible second aspect of the invention the first air heater is a secondary air heater and the invention further comprises at least one primary air heater comprising at least one process liquid heat exchanger to exchange heat between the primary air stream and the process liquid.

By heating the primary air stream with a process liquid rather than a hot flue gas stream, the problem of primary air leakage from a regenerative gas air heater is avoided, and much greater control of the heat exchange can be achieved.

The term “primary air stream” as used herein may comprise one or more primary air streams being in series, parallel or both. Embodiments of the present invention as described hereinafter may apply to a single primary air stream, each of a plurality of primary air streams, or be variable across a plurality of primary air streams. Thus, whilst the present invention is described hereinafter in relation to a single primary air stream, the invention is not limited thereto.

The primary air stream may comprise ambient air, recycle gas, or any combination or ratio extending from 0-100% thereof, optionally with the addition of one or more further components such as a near or pure oxygen stream. The requirement of the primary air stream is to at least partly assist the preparation and/or transportation of fuel into the steam generator, optionally as well as combustion support.

Where the primary air stream comprises two or more primary air streams, each primary air stream may comprise the same or different characteristics and/or composition, including but not limited to flow rate, flow volume, temperature, pressure, oxygen content and recycle gas content.

Where the primary air stream comprises two or more primary air streams, each primary air stream may be heated the same or differently, and by the same of different number of primary heat exchangers.

Where the primary air stream comprises two or more primary air streams, two or more of such air streams may be combined after heating according to the present invention either prior to, during or after their intended use or destination.

Where the primary air stream comprises two or more primary air streams, the primary air streams may be passed separately and/or in any combination to the steam generator.

In one embodiment, there are provided a plurality of primary air streams, each primary air stream for the preparation and/or transportation of a separate fuel stream into the steam generator, such as an equivalent plurality of fuel pulverisers, each passing a separate fuel stream into a boiler. In this way, there can be separate control of each primary air stream, providing greater control flexibility of the overall fuel preparation, transportation and supply into the steam generator.

The system may comprise two or more primary heat exchangers in series, parallel or both, providing heat exchange with one or more primary air streams, also being in series, parallel or both, such that they system may comprise any number of primary air streams and primary air heat exchangers in any combination thereof.

In one embodiment of the present invention, the primary air heat exchanger(s) comprise at least one high pressure heat exchanger and at least one low pressure heat exchanger. The terms “high pressure” and “low pressure” are known to the person skilled in the art in relation to steam generating processes, especially involving a steam generator such as a boiler and a feed heating/heat recovery system.

The process liquid may be provided in one or more streams. A plurality of process liquid streams may be provided in series, parallel or both, and optionally from a single source or a plurality of sources. Further optionally, the process stream may be provided in one or more process liquid circuits, optionally a plurality of separate circuits, being separate or connected, each circuit optionally passing through a separate primary air heat exchanger to exchange heat between the primary air stream and a process liquid stream.

Each process liquid stream passes through a separate primary air heat exchanger.

As the flow of a process liquid can be controlled easily, the system of the present invention can provide better control of the heating of the primary air stream. No tempering air source supply into the primary air stream may be required to achieve the correct temperature of the primary air stream prior to its use in the steam generating process. Preferably, no primary air stream gas-to-air heaters are required.

In a possible embodiment, the primary air heat exchanger(s) comprise at least one high pressure heat exchanger and at least one low pressure heat exchanger. The terms “high pressure” and “low pressure” are known as subject to the pressure downstream or upstream of the feed pump(s) respectively.

For a coal-fired steam generator or boiler, the coal is typically pulverised in one or more mills prior to its use in the boiler.

According to a further embodiment of the present invention, the primary air stream passes to one or more pulverisers after being heated by the primary air heat exchanger(s), typically to two or more pulverisers. Preferably, each primary air stream passes to a respective pulveriser.

Preferably, each primary air stream is separately heated by separate primary air heat exchanger(s).

The module of the present invention may be provided integrally as part of a steam generating process, or be added to an existing steam generating process by being installed, such as retrofitted, in the path of flue gas exhausted from an existing steam generator such as a boiler.

According to a third aspect of the present invention, there is provided a steam generation system comprising a steam generator such as a boiler and a flue gas heat recovery module as herein defined. The steam generation system preferably involves one or more steam generating processes as herein described.

The steam generation system preferably includes one or more first flow paths for directing process liquid from a steam generator feedwater stream to the process liquid economisers, and one or more second flow paths for directing process liquid from the process liquid economisers to the steam generator feedwater stream.

The steam generating system of the present invention may also include:

(d) a third flue gas conduit defining a flow path for flue gas from the first air heater to a second junction; (e) a fourth flue gas conduit defining a flow path for flue gas from the process liquid economisers to a second junction; (f) a fifth flue gas conduit defining a flow path for flue gas from the second junction; and (g) a downstream process fluid heat exchanger in the path of the fifth conduit.

The downstream process fluid heat exchanger could be used to directly or indirectly provide heat to one or more streams in a steam generating process, particularly one or more incoming air streams requiring a higher than ambient temperature for their proposed use. Such incoming air streams can include a primary air stream, a secondary air stream, or both, either individually or as a combined stream such as prior to their separation into primary and secondary air streams. This may provide further heat recovery from the flue gas, ensuring greater heat utilisation and greater efficiency of available energy in a steam generating process.

Thus, according to the present invention, there may be provided a steam generation system comprising heat recovery from exhausted flue gas in a steam generator comprising a heat recovery module in accordance with the first aspect of the invention disposed to heat a downstream process fluid to heat one of: any or the primary air flow, any or the secondary air flow, or both, of the steam generation system.

The present invention can therefore provide a steam generation system having heat recovery from exhausted flue gas from a steam generator in a steam generating process comprising:

(a) a flue gas outlet conduit defining a flow path for flue gas from a flue gas outlet of a steam generator to a flue gas conduit first junction; (b) a first flue gas conduit defining a first flow path for flue gas from the first junction to a first air heater; (c) a second flue gas conduit defining a second flow path for flue gas in parallel with the first flue gas conduit from the first junction to at least one low pressure and at least one high pressure process liquid economiser; (d) a third flue gas conduit defining a flow path for flue gas from the first air heater to a second junction; (e) a fourth flue gas conduit defining a flow path for flue gas from the process liquid economisers to a second junction; (f) a fifth flue gas conduit defining a flow path for flue gas from the second junction; and (g) a downstream process liquid heat exchanger in the path of the fifth conduit to heat a downstream process fluid to heat one of: any or the primary air flow, any or the secondary air flow, or both, of the steam generation system.

According to a fourth aspect of the present invention, there is provided a heat recovery method for recovering heat from exhaust flue gases of a steam generator comprising the following steps:

(i) dividing flue gas exhausted from a steam generator into two streams; (ii) causing a first stream to feed into a first air heater; (iii) causing a second stream to feed into at least two process liquid economisers comprising at least one low pressure and at least one high pressure process liquid economiser.

The heat recovery method preferably uses a heat recovery module as herein defined. In the heat recovery method, the first air heater is preferably a regenerative gas air heater.

Preferably the first air heater is a secondary air heater and the method comprises the further step of passing a process liquid through one or more primary air heat exchangers to exchange heat from the process liquid to the primary air steam in the primary air heat exchanger(s).

Preferably, the method of heating a primary air stream according to the present invention comprises heating the primary air stream using the embodiment of the system as hereinabove described.

The heat recovery method may further comprise the step of combining the first and second streams after the first air heater and the process liquid economisers to form a combined stream which is subsequently in direct or indirect heat exchange with one of: the primary air flow, the secondary air flow or both.

Preferably, the method for recovering heat from exhaust flue gas of a steam generator according to the present invention comprises using a steam generating system as hereindefined.

According to a fifth aspect of the present invention, there is provided a method of modification of a heat recovery system for a steam generator having a flue gas exhaust stream supplying one or more air heaters, the method comprising providing at least one low pressure and at least one high pressure process liquid economiser fluidly in parallel with the air heater(s).

The fifth aspect of the present invention in particular comprises a method of after-market modification of existing plant. The present invention is particularly suited to retrofitting. Many thermal power plants suffer high backend temperatures (that is, high flue gas temperatures at the air heater outlet) particularly when the fuel moistures vary significantly. Almost all the thermal power plants operate at high backend temperatures in the summer. By adding slip stream economisers with reasonable sizes in parallel with the air heaters modification can be made without any changes on the costly major equipment, such as FDF, IDF, AH SAH etc. The heat capacity of the boiler exhausted flue gas will better match the heat capacity of the combustion air and the slip stream feed flow in the economisers. The backend temperatures will drop significantly and the energy losses of the heat transfer process will be minimised. Therefore the cycle efficiency is improved. Retrofit can be carried out for the plants either with or without steam/water air heaters.

Steam or water air heaters are installed in most of the existing plants. They are the heat exchangers for pre-heating the air streams before entering gas air heaters. With the support of steam/water air heaters and relatively bigger economisers, the backend temperatures can drop further for improving the boiler thermal efficiency.

The method thus preferably comprises a method of retrofitting a module in accordance with the first aspect of the invention to a heat recovery system of a steam generator having a flue gas exhaust stream supplying one or more air heaters.

In a yet further aspect the invention comprises a heat recovery system of a steam generator having a flue gas exhaust stream supplying one or more air heaters with such a module retrofitted.

The present invention encompasses all combinations of various embodiments or aspects of the invention described herein. It is understood that any and all embodiments of the present invention may be taken in conjunction with any other embodiment to describe additional embodiments of the present invention. Furthermore, any elements of an embodiment may be combined with any and all other elements of any other embodiment to describe additional embodiments.

Embodiments of the present invention will now be described by way of example only, and with reference to the accompanying drawings in which;

FIG. 1 comprises a first scheme for a heat recovery module according to one embodiment of the present invention in a steam generating process;

FIG. 2 comprises the first scheme shown in FIG. 1 in an expanded steam generating process; and

FIG. 3 comprises the heat recovery module of FIG. 1 and steam generation systems according to further embodiments of the present invention.

For the purposes of this description, a single reference number will be assigned to line as well as a stream carried in that line.

Referring to the drawings, FIG. 1 shows a first scheme A for a heat recovery module for use in a steam generating process. For the first scheme A, the steam generating process includes a steam generator being a boiler 4. The boiler 4 may comprise a number of inlets and outlets for the passage of a number of streams, in particular a feedwater stream 2 thereinto, and steam therefrom. The majority of these inlets and outlets and streams are not shown in FIG. 1 for clarity purposes.

The combustion process in the boiler 4 produces exhaust flue gas, which is reduced in temperature within the boiler 4 to produce steam, before exiting the internal economiser in the boiler 4 via a flue gas outlet 13 to a flue gas outlet conduit 14. The flue gas outlet conduit 14 passes to a flue gas conduit first junction 16 which comprises a set of proportioning dampers to divide the flue gas in use into a first gas flue path in a first flue gas conduit 18 and a second flue gas path in a second flue gas conduit 20.

The first flue gas conduit 18 passes its flue gas to a first air heater, being in the first scheme A of FIG. 1 a regenerative gas air heater to heat one or more air streams able to pass therethrough. For example, the first air heater could be a secondary air pre-heater, such as a regenerative gas secondary air heater 22. In the secondary air heater 22, the heat energy of the flue gas is exchanged with a secondary air stream 24 in a manner known in the art, to provide from suitable outlets a cooler flue gas in a third flue gas conduit 26 (defining a flow path for flue gas from the secondary air heater 22 to a second junction 28), and a hotter secondary air stream 30 which can be passed, directly or indirectly, into the boiler 4 for use in the combustion of the fuel in the boiler 4 in a manner known in the art.

Additionally and/or alternatively, the first air heater in FIG. 1 could provide heat to one or more other air streams represented in FIG. 1 by line 24 a. This could include one or more primary air streams useable for a coal-fired boiler. The first air heater could therefore be a multi-sector air heater to heat both primary air and secondary air. Alternatively, the combustion air can by split into primary and secondary air streams at the outlet of airheaters. Hot primary air fan can be installed to boost primary air pressure.

The flue gas in the second flue gas conduit 20 defines a second flow path for flue gas in parallel with the first flue gas conduit 18 from the first junction 16 to one or more process liquid economisers. In the first scheme A of FIG. 1, there is shown a high pressure (process liquid) economiser 32 followed by an in-line low pressure (process liquid) economiser 34.

The high pressure and low pressure economisers 32, 34 are able to exchange heat in the flue gas originating from the boiler 4 with one or more process liquids. In the example of the present invention shown by the first scheme A of FIG. 1, the high pressure economiser 32 is provided with process liquid as a first process liquid stream 36. The process liquid of the first process liquid stream 36 may be any suitable liquid available in the steam generating process. Preferably the process liquid of the first process liquid stream 36 is a slip stream of the main feedwater stream 2 to be subsequently processed by the steam generator of the steam generating process, being the coal-fired boiler 4. This is described in more detail with reference to FIG. 2 hereinafter.

The first process liquid stream 36 is able to extract heat from the flue gas in the second flue gas conduit 20 to provide a hotter first process liquid stream 38 and a cooler flue gas stream 40, which cooler flue gas stream 40 passes via a suitable inlet into the low pressure economiser 34.

In a similar manner, the cooler flue gas stream 40 is able to provide heat to a second process liquid stream 42 passing via a suitable inlet into the low pressure economiser 34, to provide, via suitable outlets, a hotter second process liquid stream 44 and a further cooler flue gas stream which can pass along a fourth flue gas conduit 46 defining a flow path for the flue gas from the low pressure economiser 34 to the second junction 28.

From the second junction 28, there is a fifth flue gas conduit 48 defining a flow path for flue gas from the second junction 28.

A feature of the present invention is the suitable control of the division of the flue gas in the flue gas outlet conduit 14 between the first flue gas conduit 18 and the second flue gas conduit 20 to best match the heat capacity of the flue gas in the first and second flue gas paths with the heat capacities of the air flow in the first air heater, being a regenerative gas secondary air heater 22 in the first scheme A of FIG. 1, and the process liquid economisers, being the high pressure and low pressure economisers 32, 34 in the first scheme A of FIG. 1. In this way, improved cycle efficiency can be achieved by minimizing the energy losses of the heat transfer process with the minimized LMTD (Log Mean Temperature Difference) for recovering the heat from the exhausted flue gas from the boiler 4.

The use of a process liquid such as water in process liquid economisers, rather than air in gas-to-air heat exchangers, in the path of at least a portion of the exhausted flue gas from the steam generator provides significant advantages. These include the transport of a process liquid, such as around a circuit, which can be carried out with minimal pressure loss over distance, even using small pipes. Another advantage is by carefully selecting and controlling the water flow, and the temperature differentials in the first air heater and process liquid economiser(s), there can be optimising of the heat exchange to the process liquid (and thus minimize the LMTD).

In addition, as the specific heat capacity of liquids such as water is much higher than the specific heat capacity of gases such as air, the flow of the process liquid such as water can be significantly reduced while still providing the same heat exchange amount from the flue gas stream.

In particular, the use of feedwater as the process liquid for the process liquid economisers in the first scheme A of FIG. 1 improves the efficiency of the use of the heat energy of the flue gas by directly transferring heat to the feedwater and carefully selecting the flow and tap points used to provide the first and second process liquid stream 36, 42 for the slipstream(s) of feedwater used.

The schemes described above are particularly suited to retrofitting to existing plant.

The slip stream economisers can be sized to achieve the minimised backend temperatures based on the fuel sulphur contents when firing low moisture coals in the summer. A gas damper may be installed on the bypass gas duct. The bypass gas will be reduced when firing high moisture coals and/or in the winter so that the minimised backend temperatures can be maintained based on the fuel sulphur contents regardless of the fuel moistures and ambient temperatures.

With bigger slip stream economisers and the support of steam or water air heaters, more flue gas can be bypassed to slip stream economisers to further drop the backend temperature of gas airheater while maintaining the AH cold end metal temperatures above certain value to prevent corrosion caused by acid condensate.

Low temperature turbine bleed steam or feed water is used to heat the combustion air in the water/steam air heaters. One or two slip stream economisers using high and low pressure boiler feed water as cooling media may be arranged. The flow and temperature of the cooling or heating media for the slip stream economisers and the water/steam air heaters should be carefully selected to ensure the most effective energy recovery as well as cost effectiveness.

Boiler efficiency is very sensitive to the backend flue gas temperatures. The benefits of the above retrofits are very significant. However by introducing slip stream economisers, the amount of turbine bleed steam shall be reduced. With the utilization of water/steam air heaters, part of the bleed steam reduction will be trade off. The negative effect on the turbine heat rate caused by the slip stream economisers is partly compensated by the utilization of water/steam air heaters. In general, the total plant efficiency improvement is still very significant.

No modification on the existing major equipment, such as FDF, IDF, AH etc is required. The main cost will be the additional slip stream economisers. There are flexibilities on sizing the slip stream economisers. Sizing may be optimized having regard to factors such as budget, fuel prices and the operation conditions of the plant.

FIG. 2 shows the first scheme A of FIG. 1 with further detail of a steam generation system using a coal-fired boiler, in particular further detail in the primary air stream and the feedwater stream.

Coal for the boiler 4 in FIG. 2 is supplied as a coal stream 6. The coal stream 6 is typically pulverised in one or more pulverisers 8, typically 4-6 pulverisers, so as to be provided to the boiler 4 as a pulverised coal stream 10. A primary air stream 12 is provided to the one or more pulverisers 8.

In the path of the primary air stream 12 shown in FIG. 2, there is shown a first low pressure heat exchanger 50 able to exchange heat between a first heating stream 52 and the primary air stream 12 to provide a hotter primary air stream 12 a and a cooler first heating stream 54 via suitable outlets. The hotter primary air stream 12 a passes into a high pressure primary air heat exchanger 55 to be further heated by a second heating stream 56. The high pressure primary air heat exchanger 55 provides through suitable outlets a further heated primary air stream 12 b which can be passed to the one or more pulverisers 8, and a cooler second heating stream 58.

The first and second heating streams 52, 56 could be provided from the same or separate sources, and could be provided as part of the same or separate circuits. For example, the first heating stream 52 could be provided from a deaerator discharge stream, and the second heating stream could be provided from a feedwater slip stream, such as after the high pressure feedwater heaters. In a non-limiting example of the present invention, the cooler first and second heating streams 54, 58 could be directly or indirectly provided as at least part of, optionally all of, the first and second process liquid streams 36, 42.

FIG. 2 also shows a more detailed feedwater stream 2, having a first slip stream 60 at a suitable first feedwater junction 62, such as in or part of one of more of the feedwater heaters known in the art. The first slip stream 60 could be used to provide directly or indirectly process liquid for the second process liquid stream 42, using any suitable degree of separation, with the remaining feedwater stream 64 following the first feedwater junction 62.

FIG. 2 shows the return of the hotter second process liquid stream 44 as a first return stream 66 passing into the remaining feedwater stream 64. The re-combined feedwater stream 68 may undergo one or more processes such as heating, pressurising, or both, to provide a processed feedwater stream 70, from which a second feedwater slip stream 72 could be provided via a second feedwater junction 74 in the same manner as that described hereinabove for the first feedwater junction 62.

In the same manner as described above, the second slip stream 72 could be used to provide directly or indirectly process liquid for the first process liquid stream 36, using any suitable degree of separation, with the remaining feedwater stream 76 following the second feedwater junction 74. FIG. 2 shows the return of the hotter first process liquid stream 38 as a second return stream 78 passing into the remaining feedwater stream 76. The recombined feedwater stream 80 can then be passed, either directly or indirectly, into the boiler 4.

FIG. 2 shows suitable arrangements for the provision of the first and second process liquid streams 36, 42 from a feedwater stream 2, as well as a suitable arrangement for the use of first and second heating streams 56, 52 to provide heating to the primary air stream 12, optionally in conjunction with the conduits and circuits used for the process liquid economisers.

FIG. 3 shows the schemes of FIGS. 1 and 2 in combination with another steam generation system according to another embodiment of the present invention.

In FIG. 3, the flue gas in the fifth flue gas conduit 48 from the second junction 28 passes through an electrostatic precipitator (ESP) 82 and an induction fan 84, followed by passage via a suitable inlet into a downstream process fluid heat exchanger 86 in the path of the fifth conduit 48. The downstream process fluid heat exchanger 86 is able to extract any remaining available heat energy from the flue gas with a downstream process fluid in a first process circuit 100.

The first process circuit 100 comprises a process fluid in at least a first downstream process fluid conduit 102 passing via a suitable inlet into the downstream process fluid heat exchanger 86 to provide, via a suitable outlet, a hotter downstream process fluid stream 104 in a second fluid conduit 104. At a first circuit junction 106, the hotter downstream process fluid stream 104 may be divided by a suitable controller anywhere between 0-100% between a secondary air heating stream 108 and a primary air heating stream 110.

The secondary air heating stream 108 passes via a suitable inlet into a secondary air preheat exchanger 112 to provide heat exchange with an initial secondary air stream 90, so as to provide some pre-heating to the secondary air prior to the subsequent heating of the secondary air stream 24 by the regenerative gas secondary air heater 22.

Similarly, the primary air heating stream 110 can provide some pre-heating to an initial primary air stream 92 (after passage through a primary air fan 94) in a primary air preheat exchanger 114, to provide a primary air stream 12 to be subsequently heated by the first and second air heat exchangers 50, 55 as described above.

The downstream process fluid of the downstream process fluid circuit 100 may be any suitable liquid, gas or combination of same, usually at low pressure and usually circulated by one or more suitable circulation pumps in the circuit 100.

The primary and secondary air preheat exchangers 112, 114 provide cooler return streams 116, 118 respectively, which can be recombined to provide the process fluid in the first downstream process fluid conduit 102. The downstream process fluid heat exchanger 86 provides a cooler flue gas stream 88.

The initial primary air stream 92 and secondary air stream 90 can be provided from a single air source stream 94, prior to being divided by a suitable controller.

In a first alternative, the hotter downstream process fluid stream 104 in the second fluid conduit 104 passes through a heat exchanger on the single air source stream 94 to heat the primary air and secondary air prior to their division.

The downstream process fluid circuit 100 increases the use of the available heat energy in the flue gas to provide some pre-heating of the primary and secondary air streams so as to maximise the efficiency of the steam generation system shown in FIG. 3. 

1. A module for heat recovery from exhausted flue gas from a steam generator in a steam generating process comprising: (a) a flue gas outlet conduit defining a flow path for flue gas from a flue gas outlet of a steam generator to a flue gas conduit first junction; (b) a first flue gas conduit defining a first flow path for flue gas from the first junction to a first air heater; and (c) a second flue gas conduit defining a second flow path for flue gas in parallel with the first flue gas conduit from the first junction to at least two process liquid economisers comprising at least one high pressure economiser and at least one low pressure economiser.
 2. A module in accordance with claim 1 wherein the first air heater comprises at least a regenerative gas air heater.
 3. A module in accordance with claim 2 wherein the first air heater comprises at least a regenerative gas secondary air heater.
 4. A module in accordance with claim 3 further comprising at least one primary air heater comprising at least one process liquid heat exchanger to exchange heat between the primary air stream and the process liquid.
 5. A module in accordance with any one of claims 1 to 4 wherein the process liquid is water.
 6. A module in accordance with claim 5 wherein the process liquid is feedwater for a steam generator.
 7. A module in accordance with claim 6 wherein the process liquid is one or more slipstreams of the feedwater stream to be processed by the steam generator of the steam generating process.
 8. A module in accordance with any one of the preceding claims comprising one or more first process liquid conduits defining one or more flow paths for directing process liquid from a feedwater stream to the process liquid economisers.
 9. A module in accordance with any one of the preceding claims comprising one or more second process liquid conduits defining one or more flow paths for directing process liquid from the process liquid economisers to a feedwater stream.
 10. A steam generation system comprising a steam generator such as a boiler and a flue gas heat recovery module in accordance with any preceding claim.
 11. A steam generation system in accordance with claim 10 comprising one or more first flow paths for directing process liquid from a steam generator feedwater stream to the process liquid economiser(s) and one or more second flow paths for directing process liquid from the process liquid economisers to the steam generator feedwater stream.
 12. A steam generation system in accordance with claim 10 or claim 11 further comprising: (d) a third flue gas conduit defining a flow path for flue gas from the first air heater to a second junction; (e) a fourth flue gas conduit defining a flow path for flue gas from the process liquid economisers to a second junction; (f) a fifth flue gas conduit defining a flow path for flue gas from the second junction; and (g) a downstream process fluid heat exchanger in the path of the fifth conduit.
 13. A steam generation system in accordance with claim 12 further comprising using the downstream process fluid heat exchanger to provide heat to one of: the primary air flow, the secondary air flow, or both.
 14. A heat recovery method for recovering heat from exhaust flue gases of a steam generator comprising the following steps: (i) dividing flue gas exhausted from a steam generator into two streams; (ii) causing a first stream to feed into a first air heater; (iii) causing a second stream to feed into at least two process liquid economisers at least one high pressure economiser and at least one low pressure economiser.
 15. A method in accordance with claim 14 using a module as defined in any one of claims 1 to
 9. 16. A method in accordance with claim 14 or claim 15 wherein the first air heater comprises at least a regenerative air heater.
 17. A method in accordance with any one of claims 14 to 16 further comprising the step of combining the first and second streams after the first air heater and the process liquid economisers to form a combined stream which is subsequently in heat exchange with one of: the primary air flow, the secondary air flow or both.
 18. A method in accordance with one of claims 14 to 17 wherein the first air heater is a secondary air heater and the method comprises the further step of passing a process liquid through one or more primary air heat exchangers to exchange heat from the process liquid to the primary air steam in the primary air heat exchanger(s).
 19. A method of modification of a heat recovery system for a steam generator having a primary flue gas exhaust stream supplying one or more air heaters, the method comprising providing at least one high pressure and at least one low pressure process liquid economiser fluidly in parallel with the air heater(s).
 20. A method in accordance with claim 19 performed as a method of modification of existing plant to retrofit a module as defined in any one of claims 1 to
 9. 